--- Module Squre
 -- @number a first coeff
 -- @number b second coeff
 -- @number c third coeff
 -- @return first root, or nil
 -- @return second root, or imaginary root error
 -- @usage local r1, r2 = solve(1, 2, 3)
function solve (a,b,c)
     local disc = b^2 - 4*a*c
     if disc < 0 then
         return nil,"imaginary roots"
     else
        disc = math.sqrt(disc)
        return (-b + disc)/2*a,
               (-b - disc)/2*a
     end
 end
